1. Field of the Invention
The Invention relates to batteries and more particularly to high performance batteries and cells.
2. Background Art
For the past few decades there has been an accelerating effort to develop high performance batteries for applications such as electric vehicles, load levelling, solar electric energy storage, and other industrial, commercial, residential and military uses. Except for the all solid-state lithium battery, these developments involve high temperature chemical reactions at the interfaces of electrolytes and metals to achieve high current density and power. The reaction at opposing interfaces are chosen to yield highest possible open circuit voltages in order to achieve maximum stored electrical energy per pound. Unfortunately, so much weight must be invested in the structure of these batteries, to contain the violent reactions within, that the specific stored energy is much less than theoretically predicted.
FIG. 7.7 from F. D. Richardson and J. H. E. Jeffes, J. Iron Steel Inst., 160, 261 (1948), Modified by L. S. Darken and R. W. Gump, Physical Chemistry of Metals, McGraw-Hill, New York, 1953, taken from "THERMODYNAMICS OF SOLIDS"; Swalin, R. A., John Wiley & Sons, Inc. New York; Copyright 1962 is a plot of specific power vs. specific stored energy of several batteries including some of the exotic high temperature batteries under development. Two of the highest performance cells are the sodium/sulfur and the Lithium/Chlorine cells which operate at temperatures well above 1000.degree. F. Theoretically, they should have more than ten times the specific stored energy shown, but the massive structure of the electrodes and the containment material imposes very low practical limits.
Despite their high performance, none of these high temperature exotic batteries are candidate for mass production, because they are too fragile, unreliable, costly, and pose unusual safety hazards. See Kirk-Othmer, "ENCYCLOPEDIA OF CHEMICAL TECHNOLOGY" Vol. 3 on Primary and Secondary Batteries and Cells, High Temperature; and "LEAD/ACID--STILL TOP OF THE GALVANIC TRACTION PILE IN 1983" D. A. J. Rand, Journal of Power Sources, 11 (1984) 119-126.
The only modern battery that is an exception to these limitations is the all solid-state lithium battery. See "AMBIENT TEMPERATURE RECHARGEABLE LITHIUM CELLS: STATE OF THE ART: PROBLEMS AND OPPORTUNITIES; Brummer, S. B., Corp. Source-EIC Labs. Inc., Newton, Mass. Journal Vol. -u8311; Technical report, December 1982. These batteries consist of relatively thin film Lithium separated from an ion insertion compound like Titanium Disulfide, by an organic film like Polyacetylene Oxide, which can pass Lithium ions with relatively little resistance. Such cells are limited to an operating temperature slightly above room temperature because Lithium and the Organic separator would melt at higher temperatures. Nevertheless, cells and batteries have been built with specific power as high as 100 watts/lb and specific stored energy as high as 100 watt-hrs./lb, and progress continues. However, it is unlikely that such batteries will prove practical for mass production of vehicle batteries or load levelling batteries, because Lithium is a relatively rare and expensive metal. It is also a dangerous metal for humans to contact.
All of these electrochemical cells obey the laws of thermodynamics, from which is derived the Gibbs-Helmholtz relationship that gives their open-circuit voltage, EQU V.sub.oc =(.DELTA.H.degree.+dV.sub.oc /dT)/Q
where .DELTA.H.degree. is the standard heat of formation of chemicals formed when charge Q is transferred around the circuit, and T d V.sub.oc /dT is the amount of heat absorbed from the cell surroundings at temperature T. See "PRINCIPLES OF ELECTRICITY", Page, L., Ilsley Adams, N. PhD 4th Edition, D. Van Nostrand Co., Princeton, N.J. Usually the second term, the so-called entropic term, of the Gibbs-Helmholtz relationship is so small compared to the first that it is ignored in designing most electrochemical cells.
In recent years, scientific investigators have been studying the ability of semiconductors to conduct ions at high temperatures. See "SOLID IONIC CONDUCTORS", Chemical and Engineering News May 20, 1985, by Duward F. Shriver, Gregory C. Fartington. However, these studies do not anticipate the use of nonmetallic conductors as thin film separators between dissimilar metals nor do they teach the use of entropic effects to achieve thermal energy conversion, nor do they teach the use of porous material that allows the cell to breathe gases in and out. Finally, none of this art anticipates the use of humidity and other additives as practical methods for reducing the resistivity of the non-metallic separators.
It is an object of this invention to achieve high performance characteristics like that shown in the above-referenced FIG. 7.7 with thermovoltaic batteries.
Another object of this invention is to choose the material in the battery such that the battery is commonly inexpensive, easily fabricated and non hazardous.
Another objective of this invention is to describe combinations of materials that can convert substantial amounts of thermal energy in the battery environment directly into electricity.
Another objective of this invention is to describe combinations of materials that will allow a battery to chemically regenerate itself as it converts thermal energy into electricity.